Many existing optical bar code scanners utilize an object detection mechanism to determine if an object is in proximity to the optically transmissive window of the scanner. Illustratively, this object detection mechanism may be implemented using an infrared light source and an infrared detector. If an object is detected, the scanner will activate a laser beam source of a bar code detection and decoding mechanism so as to attempt a reading of one or more bar codes on the object. If, during this process, a bar code is not detected and decoded, the scanner will deactivate the laser beam source and wait for another object to enter the field of view of the object detection mechanism. Unless the object detection mechanism detects the presence of another object, the laser beam source will remain deactivated, and, in this state, it will not be possible to read any bar codes. Once the object detection mechanism detects a new object, the process of attempting to read a bar code begins again. The scanner reactivates the laser beam source, thereby permitting one or more bar codes to be detected and decoded.
Problems arise if the scanner is mounted or remains in a position such that the object detection mechanism repeatedly detects the presence of a background object, over and over again, but at no time detects the absence of this object. In such situations, the scanner will fail to detect and decode bar codes. The object detection mechanism has deactivated the laser beam source in response to the background object. The laser beam source will not be reactivated unless the object detection mechanism determines that the background object has been removed.
The foregoing scenario will arise in operational environments where a bar code scanner is aimed at a counter top, or other background object, while mounted in a stand. Examples of background objects include counter surfaces, conveyer belts, stands, brackets, and various other surfaces in and around point-of-sale terminals. Some of these objects, such as counter tops, will remain stationary with respect to the bar code scanner, whereas other objects, such as conveyer belts, will not.